There have been copying machines, facsimiles, printers and the like as examples of an image forming apparatus which uses electro photography process, and which passes transfer paper through a contact section between an image carrier and a transfer roller so as to transfer a toner image onto the transfer paper. In these image forming apparatuses, when the transfer roller is cleaned, the transfer roller is brought into contact with the image carrier. Thereafter, a positive polarity potential and a negative polarity potential are alternately applied to the transfer roller so as to transfer the toner attached to the transfer roller onto the image carrier by repulsion.
However, there have been a problem that the toner attached to the transfer roller is not fully cleaned in this electric cleaning method, so that contamination is generated in the top end and the back end of transfer materials. Accordingly, a method has been proposed, in which a circumferential speed of the image carrier and the transfer roller is changed between during transfer operation and during cleaning operation, as in an image forming apparatus disclosed in JP 08-272233 A.
FIG. 10 shows parts including the image carrier and the transfer roller in the conventional image forming apparatus disclosed in JP 08-272233 A. In FIG. 10, during cleaning operation of the transfer roller 1, a photoconductor 2 is charged to a negative polarity by the charging roller 3, so that optical write by an exposure means 4 is not performed. Also, a developing roller of a developing device 5 is not driven, and transfer materials P are not fed. In this state, the transfer roller 1 is rotated for a predetermined time in synchronization with the photoconductor 2, while a cleaning bias (bias having the same polarity as regularly-charged toner) is applied to the transfer roller 1 from a power supply 6. With the application of the cleaning bias, regularly-charged toner adhering to the transfer roller 1 is transferred onto the photoconductor 2, by which the transfer roller 1 is cleaned. In addition, when a bias having an opposite polarity to the polarity of the regularly-charged toner is applied to the transfer roller 1, the reversely-charged toner adhering to the transfer roller 1 is cleaned, thereby resulting in further enhanced cleaning performance.
In this case, the rotational speed of a main motor 8 is changed by a control means 7 so that the circumferential speed of the photoconductor 2 and the transfer roller 1 in cleaning operation is made slower than the circumferential speed of the photoconductor 2 and the transfer roller 1 in transfer operation. In other words, the photoconductor 2 and the transfer roller 1 are not given different circumferential speeds to each other, but given a relatively constant circumferential speed, and the photoconductor 2 and the transfer roller 1 are integrally changed in the circumferential speed between during the transfer operation and during the cleaning operation. Thereby, a higher cleaning performance can be obtained than that in the case where the photoconductor 2 and the transfer roller 1 are unchanged in the circumferential speed between the cleaning operation and the transfer operation.
In FIG. 10, a symbol “a” denotes a cleaning unit for the photoconductor 2, and “b” denotes a fixing device. Also, a symbol “c” denotes a power supply for the developing device 5, and “d” denotes a power supply for the charging roller 3.
The conventional image forming apparatus disclosed in JP 08-272233 A has a following problem.
When the photoconductor 2 and the transfer roller 1 are integrally changed in the circumferential speed from the transfer operation to the cleaning operation, a foam section of the transfer roller 1 is deformed on the surface of the transfer roller 1. This deformation allows toner within the foam section to be discharged from the foam section, so that cleaning is performed. However, the amount of deformation in the foam section is small since the relative circumferential speed between the photoconductor 2 and the transfer roller 1 is constant, which causes such a problem that it is hard to remove the toner residing in a deep portion of the foam section.